The present invention generally relates to an automatic implantable atrial defibrillator for delivering cardioverting or defibrillating voltage to the atria of a patient and which is capable of providing effective cardioversion with reduced discomfort to the patient. The present invention is particularly directed to such an atrial defibrillator which includes a cardioverting output voltage limiter for simulating output voltage waveforms provided by storage capacitors of greater capacity than that actually delivering the cardioverting voltage.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnate blood flow as a result of prolonged atrial fibrillation. Patients afflicted with atrial fibrillation generally experience palpitations of the heart and reduced cardiac output. This often leads to dizziness or, in extreme cases, to loss of consciousness.
Atrial fibrillation is often corrected by external defibrillation of the type well known in the art. This treatment involves applying a relatively large quantity of electrical energy to the heart with external skin surface electrodes. The energy is applied in synchronism with a detected R wave (electrical activation) of the heart. The treatment is very painful and can necessitate hospitalization for as many as a few days. Unfortunately, most often, it only provides temporary relief, lasting but a few weeks.
Drugs are available for reducing the incidents of atrial fibrillation. However, such drugs have many side effects. Also, many patients are resistant to them which greatly reduces their therapeutic effect.
In order to negate the need for external defibrillation and drug therapy, implantable atrial defibrillators have been proposed to provide relief for patients suffering from this cardiac arrhythmia. Two such defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these defibrillators required the patient to recognize the symptoms of atrial fibrillation. One defibrillator required a visit to a physician for activating the defibrillator. The other defibrillator required the patient to activate the defibrillator with a magnet from external to the patient's skin.
It is preferable that an implantable cardiac device, such as an atrial defibrillator, be truly automatic. In order for an implantable atrial defibrillator to be truly automatic, it must be able to accurately detect atrial fibrillation and then safely apply cardioverting voltage to the atria to convert the same to normal sinus rhythm (NSR).
Detection of atrial fibrillation is a two-part process. First, an atrial defibrillator must be able to sense activity of the heart, such as atrial activity. One atrial defibrillator having such capability is fully disclosed in Jin et al., U.S. Pat. No. 5,267,559 which issued on Dec. 7, 1993 for ATRIAL DEFIBRILLATOR AND METHOD FOR PROVIDING ATRIAL SENSING and which is assigned to the assignee of the present invention and incorporated herein by reference.
After heart activity, such as atrial activity, is sensed, an atrial fibrillation detector then must determine if the sensed heart activity satisfies a fibrillation criteria. One such detector is fully disclosed in co-pending U.S. application Ser. No. 08/233,251, filed Apr. 26, 1994 in the names of Harley G. White and Joseph M. Bocek for SELECTIVE CARDIAC ACTIVITY ANALYSIS ATRIAL FIBRILLATION DETECTION SYSTEM AND METHOD AND ATRIAL DEFIBRILLATOR UTILIZING SAME. Another such detector is fully disclosed in co-pending U.S. application Ser. No. 08/278,055, filed Jul. 20, 1994 in the names of Jaeho Kim and Harley G. White for SYSTEM AND METHOD FOR REDUCING FALSE POSITIVES IN ATRIAL FIBRILLATION DETECTION. Both of the aforementioned co-pending applications are assigned to the assignee of the present invention and incorporated herein by reference.
Each of the aforementioned co-pending applications discloses a preferred embodiment of an atrial fibrillation detector wherein atrial cardiac events are detected from sensed atrial activity. Further, each of these preferred embodiments includes ventricular activity sensing and detection of R waves. Atrial fibrillation is detected from atrial activity occurring between detected R waves.
Once atrial fibrillation is detected, it is then necessary to apply a cardioverting voltage pulse to the atria to return the heart to NSR. To that end, a storage capacitor is charged to a voltage and then discharged to apply the cardioverting voltage to the heart. To assure that the cardioverting voltage is safely applied to the atria, it is preferred that the capacitor is discharged in synchronism with a detected R wave. To that end, co-pending U.S. application Ser. No. 08/259,476 filed Jun. 14, 1994 in the name of Harley G. White for CARDIOVERSION SYNCHRONIZATION SYSTEM AND METHOD FOR AN ATRIAL DEFIBRILLATOR, which is assigned to the assignee of the present invention and incorporated herein by reference discloses a synchronization system which includes two ventricular sense channels and requires that an R wave be sensed in both channels before the voltage may be applied. In addition, other synchronization criteria may be required to be satisfied such as a minimum interval criteria as described, for example, in Adams, et al., U.S. Pat. No. 5,207,219, which issued on May 4, 1993 for ATRIAL DEFIBRILLATOR AND METHOD FOR PROVIDING INTERVAL TIMING PRIOR TO CARDIOVERSION, and which is assigned to the assignee of the present invention and incorporated herein by reference.
Hence, as can be seen from the foregoing, an automatic atrial defibrillator must reliably sense heart activity, reliably detect the need for cardioversion, and safely apply cardioverting voltage to the atria of the patient's heart. In order to provide reasonable assurance that the cardioverting voltage will indeed successfully cardiovert the atria, the voltage applied should have a peak value above a determined minimum peak value required to effectively cardiovert the atria. That voltage level is commonly referred to as the defibrillation threshold.
The peak voltage required to effectively cardiovert increases as the capacitor discharge time or pulse width decreases. The capacitor discharge rate increases (resulting in shorter capacitor discharge times) as the capacitor capacitance decreases. Hence, larger capacitances can discharge over a longer time period than smaller capacitances. Also as a result, larger capacitances require a lower peak voltage to effectively cardiovert.
Since patients suffering from atrial fibrillation will be conscious during cardioversion (unlike patients suffering from ventricular fibrillation), perceived discomfort or pain caused by the cardioversion becomes an issue. Obviously, the less discomfort a patient experiences as a result of cardioversion the better.
Discomfort or pain resulting from cardioversion is a nervous system response to the discharged voltage. The physiologic basis for this is that nerve tissue has a much faster membrane time constant than cardiac muscle. Therefore, shorter and higher peak voltage discharges create more pain or discomfort than do longer and lower peak voltage discharges.
The logical conclusion from the foregoing would be to use as high a capacitance as possible to cardiovert the atria. This would result in the longest discharge time, the lowest required peak voltage, and the least discomfort for successful cardioversion. Unfortunately, the size of an implantable device is also of importance. Higher capacitance values require capacitors of larger size. Obviously, a point is reached where capacitor size becomes impractical.
For atrial defibrillation, it has been determined that for many patients, a capacitor having a capacitance in the range of 70 to 100 microfarads (.mu.F) and more particularly 80 .mu.F with a total discharge time of six milliseconds is a suitable choice in terms of voltage threshold and capacitor size. Some patients, however, may experience discomfort at their voltage threshold. For these patients, a longer discharge time and lower voltage threshold would be more desirable. Unfortunately, this would require a higher storage capacitance value and hence a larger sized capacitor which may not be practical.
The present invention provides an elegant solution to this problem. In accordance with its broader objectives, the present invention permits a capacitor to be discharged for a period of time which is longer than it normally would be discharged. It also, at the same time, permits a lower voltage to be applied to the patient's heart to achieve successful cardioversion. In doing so, the capacitance value, and hence the capacitance size, need not be increased.